expanded info on fail on check + missing apt-python

it now explains that apt module will auto install

lib/ansible/modules/packaging/os/apt.py

@@ -580,7 +580,7 @@ def main():
 
     if not HAS_PYTHON_APT:
         if module.check_mode:
-            module.fail_json(msg="python-apt must be installed to use check mode")
+            module.fail_json(msg="python-apt must be installed to use check mode. If run normally this module can autoinstall it")
         try:
             module.run_command('apt-get update && apt-get install python-apt -y -q --force-yes', use_unsafe_shell=True, check_rc=True)
             global apt, apt_pkg